back at it !
I got my LCD microscope out and tried to reverse-engineer the original board. I should have done that to start with.
I drew that circuit on www.falstad.com (this link takes you to my actual circuit) and got several kV out of the simulation lol.
I used the original transformer: i could only measure resistance, so 5,5ohms on primary side, and 195ohms on secondary.
First transistor hooked to wave generator is J3Y. I replaced it with a BC547 (the "copy-paste" engineer hidden inside my head told me it's fairly close lol).
Second transistor is labeled J41CG and seems pretty strong to me so i replaced it with TIP120 .

Next I put all these components onto the breadboard, and load my favourite Nano with this sketch :
const int buzzerPin = 9;
int i = 0;
unsigned long duration = 1000;
void setup() {
pinMode(buzzerPin, OUTPUT);
}
void loop() {
for (i = 1000; i < 5000; i++) {
tone(buzzerPin, i, duration);
}
for (i = 5000; i > 1000; i--) {
tone(buzzerPin, i, duration);
}
}
That is ever so LOUD !!
So this is plenty good enough for my little project !
next step